Deputy National Security Adviser Ben Rhodes announced the decision on Thursday, saying a new assessment that Assad had used chemical weapons against his own people had “changed the calculus” of the US president.

“This is an issue that we’ve been following very closely,” Rhodes said. “The Assad regime has used chemical weapons – sarin – on a small scale multiple times over the past year.”

The chemical attacks killed 100 to 150 people, the US reported, though the White House said that data on casualties is incomplete.

The US arms, which will not include anti-aircraft weaponry, will go to fighters loyal to rebel Gen. Salim Idris, who has slowly gained the confidence of Western allies.

The administration said a decades-old “red line” has been crossed – an international standard against the use of chemical weapons – and that any further “potential response” of the United States would be mulled over in the coming days.

“We’ve prepared for many contingencies, consistent with our own national interests,” Rhodes said.

Officials speaking off record to reporters since the announcement have explained that arriving at the new policy was a long, complicated process.

Even before a series of meetings in the Oval Office last week, in which the president found himself frustrated with his options, several sessions in the White House Situation Room revealed an increasingly bleak picture for the rebel cause.

The Wall Street Journal reported a tick-tock of the decision that suggests Obama used intelligence findings on chemical weapons as political cover for more active involvement, motivated by other factors – notably, increased involvement in the Syrian civil war by Iran and Hezbollah.

The White House orchestrated the rollout “to justify the decision, announced today, to arm the Syrian opposition, and to restore US credibility by finally acting on our red line regarding chemical weapons use,” Michael Eisenstadt, director of the Military and Security Studies Program at the Washington Institute for Near East Policy, told The Jerusalem Post.

The announcement caught Pentagon brass by surprise, as did the reported consideration of a no-fly zone, which sources say the US military opposed due to the cost, risks and questions about how effective it would be.

The Obama administration has been considering a partial no-fly zone based in Jordan, where Patriot missile batteries and 5,000 US troops are engaged in a military exercise through June 20.

After that date, at the discretion of the White House, Secretary of Defense Chuck Hagel could notify Congress that he intends to keep troops and military equipment in the country, either for engagement or for contingencies.

That decision is expected in the coming days.

United Nations Secretary-General Ban Ki-moon seems unlikely to support further intervention.

“There is no military solution to this conflict, even if both the [Syrian] government and the opposition, and their supporters, think there can be,” Ban said on Friday.

“The military path points directly toward the further disintegration of the country, destabilization of the region and inflammation of religious communal tensions,” he added.

Ban confirmed he had received documents from American authorities on Friday detailing their findings on chemical weapons use in Syria.

In his statement, Ban made clear that intelligence reports from any member state are appreciated, but that they are viewed with skepticism, and perhaps come with bias.

“The validity of any information on the alleged use of chemical weapons cannot be ensured,” Ban said. “Our goal remains a fully independent and impartial inquiry.”

UN investigators have yet to gain access to sites where chemical attacks are suspected to have taken place.

Outgoing ambassador to the UN Susan Rice said the American letter to Ban’s office explains that “independent streams of information” brought US intelligence agencies to a high-confidence assessment that chemical weapons had indeed been used on several occasions, and implied that sarin was not the only kind employed.

She called Hezbollah’s entry into the fight a “dramatic escalation” of deep concern to the White House.

“We believe that the Assad regime maintains control of these weapons,” Rice said. “We have no reliable, corroborated reporting to indicate that the opposition has acquired or used chemical weapons.”

She added that no decision had been made on whether the US will impose a nofly zone.